Mechanism of protection by zinc against mercuric chloride toxicity in rats: effects of zinc and mercury on glutathionine metabolism

Amelioration of mercuric chloride-induced physiologic and histopathologic alterations in rats using vitamin E and zinc chloride supplement

The drastic effects of mercuric chloride and the protective efficiency of vitamin E and zinc chloride co-supplementation were clearly investigated in this study. Male rats were divided into four groups. The first was the control. The second received vitamin E (100 mg/kg) and zinc chloride (30 mg/kg) daily. In comparison, the third received mercuric chloride (1 mg/kg) daily, and the fourth received the same mercuric chloride dose supplemented with the same vitamin E and zinc chloride doses. Mercury promotes a significant decline in body weight. It causes a considerable reduction in total red blood cells (RBCs) count and hemoglobin concentration; however, white blood cells (WBCs) increased significantly. Significant mercury-induced elevations in hepatic and renal functions were observed. Mercury induced substantial reductions in catalase (CAT) and superoxide dismutase (SOD). Mercury caused apoptotic DNA fragmentation. It induced degeneration and necrosis in the liver and kidney. It induced necrosis, leukocyte infiltration and blood vessel congestion in the cerebral cortex. Shrinkage and deterioration of Purkinje cells of the cerebellum were observed in response to mercuric chloride toxicity. Mercuric chloride enhanced shrinking in seminiferous tubules and Leydig cells. It reduced sperm count, sperm motility, and testosterone concentration; however, it promoted abnormal sperm morphology. Administration of vitamin E and zinc chloride showed marked improvement in different parameters under investigation, however, further research is needed to determine fate of mercury.

The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

Biliary and hepatic metallothionein, metals and trace elements in environmentally exposed neotropical cichlids Geophagus brasiliensis

One of the metal detoxifying mechanisms that occurs in fish is metallothionein (MT) induction and metal binding. Hepatic MT induction has been well described, but biliary MT metal detoxification has only recently been described in fish. In this scenario, metal-metal interactions have been increasingly evaluated to further understand the behavior of these contaminants regarding homeostasis and biological functions, as well as their toxic effects. Studies, however, have been mainly conducted concerning the elemental pair Se-Hg, and scarce reports are available concerning other metal pairs. Therefore, this study aimed to evaluate biliary and hepatic MT metal detoxification mechanisms in a territorial neotropical cichlid, Geophagus brasiliensis. Fish were sampled from the anthropogenically impacted estuarine Rodrigo de Freitas Lagoon, located in Southern Rio de Janeiro, and trace elements and MT were determined by inductively coupled plasma mass spectrometry (ICP-MS) and UV-Vis spectrophotometry, respectively, in fish liver and bile. MT in bile were significantly lower than in liver. Significant differences between bile and liver were observed for many trace elements, and, although most were higher in liver, Cd and Ni were significantly higher in bile, indicating efficient excretion from the body via the biliary route. A significant correlation was observed between MT and Fe in bile, and between MT in liver and Cu and Zn in bile. Molar ratio calculations demonstrated protective elements effects against Al, As, Cd, Hg, Pb and V in both bile and liver, as well as some novel interrelationships, indicating the importance of these investigations regarding the elucidation of element detoxifying mechanisms. Furthermore, investigation of other elemental associations may aid in decision-making processes regarding environmental contamination scenarios linked to public health.

Oxidative stress induced by chromium (VI) in bone of suckling rats

Exposure to hexavalent chromium Cr(VI) compounds is of concern in many Cr-related industries and their surrounding environments. K2Cr2O7 is widely recognized as an animal and human carcinogen, mutagen, and teratogen. The present study investigated the bone maturity of suckling rats whose mothers were treated with K2Cr2O7. Experiments were carried out on female Wistar rats given 700 ppm of K2Cr2O7 in their drinking water from the 14th day of pregnancy until day 14 after delivery. Exposing dams to K2Cr2O7 caused disorders in the bone of their progeny. As corollary to this, malondialdehyde levels increased, while glutathione, a non-protein thiol and vitamin C decreased. Alteration of the antioxidant system in the treated group was also confirmed by the significant decline of superoxide dismutase, catalase, and glutathione peroxidase activities. Furthermore, K2Cr2O7 induced changes in bone mineralization, especially calcium and phosphorus levels, which decreased. Whereas, in plasma and urine, they increased and decreased inversely. These results suggest that K2Cr2O7 accelerated bone resorption activity. In fact, in treated pups, total tartrate-resistant acid phosphatase, which reflected bone resorption, was enhanced while total alkaline phosphatase, which reflected bone formation, was reduced. The impairment of bone function was corresponded histologically.

Antioxidant defenses and lipid peroxidation in the cerebral cortex and hippocampus following acute exposure to malathion and/or zinc chloride

This study investigates the effects of acute exposure to organophosphate insecticide malathion (250 mg/kg, i.p.) and/or ZnCl2 (5 mg/kg, i.p.), with the following parameters: lipid peroxidation and the activity of acetylcholinesterase (AChE), glutathione reductase (GR), glutathione S-transferase (GST), glutathione peroxidase (GPx), glucose-6-phosphate dehydrogenase (G6PDH), and the levels of total glutathione (GSH-t) in the hippocampus and cerebral cortex of female rats. Malathion exposure elicited lipid peroxidation and reduced AChE activity in the cerebral cortex and hippocampus. It also reduced the activity of GR and GST, and increased G6PDH activity in the cerebral cortex, without changing the levels of GSH-t and GPx activity. ZnCl2 exposure reduced AChE activity and caused a mild pro-oxidative effect, since lipid peroxidation was increased in the hippocampus. ZnCl2, individually or in combination with malathion, caused a reduction in GR and GST activity in the cerebral cortex. Malathion and/or ZnCl2 did not change the GSH-t levels. Moreover, ZnCl2 prevented the increase in G6PDH activity caused by malathion. It showed that ZnCl2 had little effect against the changes induced by malathion. In fact, zinc itself produced pro-oxidant action, such as the reduction in the activity of the antioxidant enzymes GR and GST.

Glutamate-cysteine ligase attenuates TNF-induced mitochondrial injury and apoptosis

Glutathione (GSH) is important in free radical scavenging, maintaining cellular redox status, and regulating cell survival in response to a wide variety of toxicants. The rate-limiting enzyme in GSH synthesis is glutamate-cysteine ligase (GCL), which is composed of catalytic (GCLC) and modifier (GCLM) subunits. To determine whether increased GSH biosynthetic capacity enhances cellular resistance to tumor necrosis factor-alpha- (TNF-alpha-) induced apoptotic cell death, we have established several mouse liver hepatoma (Hepa-1) cell lines overexpressing GCLC and/or GCLM. Cells overexpressing GCLC alone exhibit modest increases in GCL activity, while cells overexpressing both subunits have large increases in GCL activity. Importantly, cells overexpressing both GCL subunits exhibit increased resistance to TNF-induced apoptosis as judged by a loss of redox potential; mitochondrial membrane potential; translocation of cytochrome c to the cytoplasm; and activation of caspase-3, caspase-8, and caspase-9. Analysis of the effects of TNF on these parameters indicates that maintaining mitochondrial integrity mediates this protective effect in GCL-overexpressing cells.

Copper-inducible transcription: regulation by metal- and oxidative stress-responsive pathways

Although copper is an essential metal, it is capable of catalyzing the formation of reactive oxygen species that can cause intracellular oxidative damage. We investigated the hypothesis that metal- and oxidative stress-responsive signal transduction pathways mediate the cellular and molecular responses associated with copper exposure. Transient transfection assays using COS-7 cells and mouse metallothionein-I (MT-I) or rat NAD(P)H:oxidoreductase 1-based reporter genes demonstrate that copper activates transcription via metal and antioxidant response elements. Concomitant with copper exposures is a decrease in the level of total glutathione and an increase in oxidized glutathione. Depletion of glutathione, before copper exposure, increases metal- and oxidative stress-inducible transcription and cytotoxicity. Pretreatment with the reactive oxygen scavengers aspirin or vitamin E provides partial protection against copper toxicity and reduces inducible transcription. Experiments using signal transduction inhibitors and a metal transcription factor (MTF)-1 null cell line demonstrate that copper-inducible MT-I transcription is regulated by protein kinase C and mitogen-activated protein kinase signaling pathways and requires MTF-1. The results of these studies indicate that copper activates transcription through both metal- and oxidative stress-responsive signal transduction pathways.

Environmental exposure to mercury and its toxicopathologic implications for public health

Mercury is a toxic and hazardous metal that occurs naturally in the earth's crust. Natural phenomena such as erosion and volcanic eruptions, and anthropogenic activities like metal smelting and industrial production and use may lead to substantial contamination of the environment with mercury. Through consumption of mercury in food, the populations of many areas, particularly in the developing world, have been confronted with catastrophic outbreaks of mercury-induced diseases and mortality. Countries such as Japan, Iraq, Ghana, the Seychelles, and the Faroe Islands have faced such epidemics, which have unraveled the insidious and debilitating nature of mercury poisoning. Its creeping neurotoxicity is highly devastating, particularly in the central and peripheral nervous systems of children. Central nervous system defects and erethism as well as arrythmias, cardiomyopathies, and kidney damage have been associated with mercury exposure. Necrotizing bronchitis and pneumonitis arising from inhalation of mercury vapor can result in respiratory failure. Mercury is also considered a potent immunostimulant and -suppressant, depending on exposure dose and individual susceptibility, producing a number of pathologic sequelae including lymphoproliferation, hypergammaglobulinemia, and total systemic hyper- and hyporeactivities. In this review we discuss the sources of mercury and the potential for human exposure; its biogeochemical cycling in the environment; its systemic, immunotoxic, genotoxic/carcinogenic, and teratogenic health effects; and the dietary influences on its toxicity; as well as the important considerations in risk assessment and management of mercury poisoning.

Evidence of zinc protection against 2,5-hexanedione neurotoxicity: correlation of neurobehavioral testing with biomarkers of excretion

Risk prevention of human exposure against n-hexane neurotoxicity is relevant towards the protective measures to be proposed in occupational toxicology. Metabolic studies have identified 2,5-hexanedione (2,5-HD) as the main neurotoxic metabolite of n-hexane, which reacts with amino groups of lysine in axonal neurofilaments forming 2,5-dimethylpyrrole adducts, which are responsible for n-hexane neurotoxicity. In the present study, we have investigated the interaction of zinc with 2,5-HD, by correlating the decrease of pyrrole derivatives excretion with changes of neurobehavioral effects. Two subchronic experiments (11 and 8 weeks of exposure) were performed in Wistar rats exposed to different doses of 2,5-HD (200, 400 mg/kg per day) and to the mixture of 2,5-HD + zinc acetate (200 + 300 mg/kg per day) and (400 + 500 mg/kg per day). The results obtained show a significant increase in the excretion of pyrroles in the groups exposed to 2,5-HD alone as compared to controls, and a significant decrease in the excretion of pyrrole derivatives in the groups of rats co-exposed to 2,5-HD + zinc acetate when compared to the rats exposed to 2,5-HD alone. These biochemical changes were immediately evident after the first day of exposure. Simultaneously, neurobehavioral testing (rearing and ambulation in open field) was performed weekly in the same groups of rats. The results demonstrated a significant decrease in neurobehavioral dysfunction in rats co-exposed to 2,5-HD and zinc acetate. At the end of the exposure period, pyrroles levels returned to control values progressively, and the recovery of the neurotoxic effects was gradually established depending on the dose of exposure. The results suggest that zinc is a potential chemo-protector against 2,5-HD neurotoxicity which was identified by neurobehavioral testing. Moreover, pyrrole derivatives are good predictive biochemical biomarkers of 2,5-HD exposure and could be used as a complementary tool to characterize its neurotoxic effects.

Retrovirally expressed metal response element-binding transcription factor-1 normalizes metallothionein-1 gene expression and protects cells against zinc, but not cadmium, toxicity

Metal response element (MRE) transcription factor-1 (MTF1), a member of the Cys2-His2 class of zinc-finger transcription factors, is best known for its robust transcriptional regulation of mammalian metallothionein (MT) genes. MTF1 is also believed to play a generalized role in regulating genes involved in protection against heavy metals and oxidative stress. MTF1 binding to MRE motifs is regulated by changes in intracellular zinc (Zn(2+)) concentration. Molecular dissection of MTF1 has been hindered by its high constitutive trans-activity following transient transfection and the failure of these systems to examine genes packaged in native chromatin. In developing a system to avoid these problems, we employed a high-efficiency retroviral transduction system to reintroduce MTF1 into mouse Mtf1(-/-) knockout cells (dko7). Electrophoretic mobility shift assays demonstrated that MTF1 retrovirally transduced dko7 cells (MTF1dko7) possess levels of inducible MTF1-MRE binding activity similar to that seen in mouse hepatoma Hepa-1 cells, and MTF1 binding could be modulated over a 20-fold range by varying the concentration of Zn(2+) present in the culture medium. The dko7 cells exhibited no change in Mt1 gene expression upon Zn(2+) or cadmium (Cd(2+)) treatment; in contrast, in MTF1dko7 cells, Zn(2+) or Cd(2+) induced MT1 mRNA accumulation in a dose-dependent manner. Interestingly, MTF1dko7 cells showed resistance to Zn(2+) toxicity, but negligible resistance to Cd(2+). Concomitantly, MT1 protein levels in MTF1dko7 cells were inducible to the same degree as that in Hepa-1 cells when treated with Zn(2+), but not with Cd(2+). Together, our studies suggest that MTF1-mediated regulation of gene expression is sufficient to protect cells against Zn(2+) toxicity and may be necessary but not sufficient to protect cells against Cd(2+) toxicity.

A role for mitogen-activated protein kinases in the etiology of diabetic neuropathy

The onset of diabetic neuropathy, a complication of diabetes mellitus, has been linked to poor glycemic control. We tested the hypothesis that the mitogen-activated protein kinases (MAPK) form transducers for the damaging effects of high glucose. In cultures of adult rat sensory neurons, high glucose activated JNK and p38 MAPK but did not result in cell damage. However, oxidative stress activated ERK and p38 MAPKs and resulted in cellular damage. In the dorsal root ganglia of streptozotocin-induced diabetic rats (a model of type I diabetes), ERK and p38 were activated at 8 wk duration, followed by activation of JNK at 12 wk duration. We report activation of JNK and increases in total levels of p38 and JNK in sural nerve of type I and II diabetic patients. These data implicate MAPKs in the etiology of diabetic neuropathy both via direct effects of glucose and via glucose-induced oxidative stress.

Interference of heavy metal cations with fluorescent Ca2+ probes does not affect Ca2+ measurements in living cells

In studies about the effects of heavy metals on intracellular Ca2+, the use of fluorescent probes is debated, as metal cations are known to affect the probe signal. In this study, spectrofluorimetric experiments in free solution, using Fluo-3 and Fura-2, showed that Zn2+ and Cd2+ enhanced the probe signal, Cu2+ quenched it, and Hg2+ had no effect. Addition of GSH prevented most of these effects, suggesting the occurrence of a similar protective role in living cells. Digital imaging of living mussel haemocytes loaded with Fura-2/AM or Fluo-3/AM showed that Hg2+, Cu2+ and Cd2+ induced a rise in probe fluorescence, whereas up to 200 microM Zn2+ had no effect. In particular, Cd2+ produced the strongest probe signal rise in free solution, but the lowest fluorescence increase in cells. Probe calibration yielded [Ca2+]i values characteristic of resting levels in control and Zn2+-exposed cells, and, as expected, indicated Ca2+ homeostasis impairment in cells exposed to Cd2+, Cu2+ and Hg2+. Our results show that Ca2+ probe responses to heavy metals in living cells are completely different from those obtained in free solution, indicating that fluorescent probes can be a suitable tool to record the effects of heavy metals on [Ca2+]i.

The influence of nutrition on methyl mercury intoxication

This article reviews progress in the research of methyl mercury (MeHg) and nutrient interactions during the past two decades. Special emphasis is placed on the following three major areas: a) effects on kinetics, b) effects on toxicity, and c) possible mechanisms. Dietary information is not usually collected in most epidemiologic studies examining of the effects of MeHg exposure. However, inconsistency of the MeHg toxicity observed in different populations is commonly attributed to possible effects of dietary modulation. Even though the mechanisms of interaction have not been totally elucidated, research in nutritional toxicology has provided insights into the understanding of the effects of nutrients on MeHg toxicity. Some of this information can be readily incorporated into the risk assessment of MeHg in the diets of fish-eating populations. It is also clear that there is a need for more studies designed specifically to address the role of nutrition in the metabolism and detoxification of MeHg. It is also important to collect more detailed dietary information in future epidemiologic studies of MeHg exposure.

Heavy metals mercury, cadmium, and chromium inhibit the activity of the mammalian liver and kidney sulfate transporter sat-1

Heavy metal intoxication leads to defects in cellular uptake mechanisms in the mammalian liver and kidney. We have studied the effects of several heavy metals, including mercury, lead, cadmium, and chromium (at concentrations of 1 to 1000 microM), on the activity of the mammalian sulfate transporter sat-1(2) in Xenopus oocytes. sat-1 encodes a sulfate/bicarbonate anion exchanger expressed in the rat liver and kidney. Mercury (10 microM) strongly inhibited sat-1 transport by reducing Vmax by eightfold but not its Km for inorganic sulfate (Si). Lead (up to 1 mM) was unable to significantly inhibit sat-1 transporter activity. Cadmium (500 microM) showed weak inhibition of sat-1 transport by decreasing only sat-1 Vmax. Chromium (100 microM) strongly inhibited sat-1 transport by reducing Km for Si by sevenfold, most probably by binding to the Si site, due to the strong structural similarity between the CrO2-4 and SO2-4 substrates. This study presents the first characterization of heavy metal inhibition of the hepatic and renal sulfate/bicarbonate transporter sat-1, through various mechanisms, which may lead to sulfaturia following heavy metal intoxication.

Renal Na-Si cotransporter NaSi-1 is inhibited by heavy metals

Heavy metal intoxication leads to a number of reabsorptive and secretory defects in renal transport systems. We have studied the effects of several heavy metals on the expression of the renal Na-Si cotransporter NaSi-1. NaSi-1 cRNA was injected into Xenopus oocytes, and Na-Si cotransport activity was measured in the presence of mercury, lead, cadmium, or chromium. Mercury strongly inhibited NaSi-1 transport irreversibly by reducing both maximal velocity (Vmax) and Michaelis constant (Km) for inorganic sulfate (Si). Lead inhibited NaSi-1 transport reversibly by decreasing Vmax but not Km for Si. Cadmium showed weak reversible inhibition of NaSi-1 transport by decreasing only NaSi-1 Vmax. Chromium strongly inhibited NaSi-1 cotransport reversibly by reducing Km for Si by sevenfold, most probably by binding to the Si site, due to the strong structural similarity between the CrO4(2-) and SO4(2-) substrates. In conclusion, this study presents an initial report demonstrating heavy metals inhibit renal brush border Na-Si cotransport via the NaSi-1 protein through various mechanisms and that this blockade may be responsible for sulfaturia following heavy metal intoxication.

Degree of peroxidative status in neuronal tissues by different routes of inorganic mercury administration

Mercuric chloride was administered by three routes--subcutaneous, intramuscular and intraperitoneal to adult female Wistar rats. The peroxidative status of the cerebral cortex, cerebellum and the sciatic nerves were studied. Enhanced levels of lipid peroxides indicate progressing cellular injury. All the experimental groups show high levels of reduced glutathione and increased activities of glutathione peroxidase, superoxide dismutase and catalase. Neurotoxic status was more pronounced in intramuscularly administered mercuric chloride, followed subsequently by intraperiotoneal and subcutaneous routes of administration.

Understanding renal toxicity of heavy metals

The mechanisms by which metals induce renal injury are, in general, poorly understood. Characteristic features of metal nephrotoxicity are lesions that tend to predominate in specific regions of the nephron within specific cell types. This suggests that certain regions of the nephron are selectively sensitive to specific metals. Regional variability in sensitivity could result from the localization of molecular targets in certain cell populations and/or the localization of transport and binding ligands that deliver metals to targets within the nephron. Significant progress has been made in identifying various extracellular, membrane, and intracellular ligands that are important in the expression of the nephrotoxicity of metals. As an example, mercuric chloride induces a nephropathy that, at the lowest effective doses, is restricted primarily to the S3 segment of the proximal tubule, with involvement of the S2 and S1 segments at higher doses. This specificity appears to be derived, at least in part, from the distribution of enzymes and transport proteins important for the uptake of mercury into proximal tubule cells: apical gamma-glutamyltranspeptidase and the basolateral organic anion transport system. Regional distributions of transport mechanisms for binding proteins appear to be important in the expression of nephrotoxicity of metals. These and other new research developments are reviewed.

[Zinc--update of an essential trace element]

Since the recognition of zinc as an essential trace element in man and animals there has been a remarkable progress in our knowledge of the role of zinc in nutritional physiology, biology and medicine during the last few decades. Highlights in zinc research, mechanisms and homeostatic regulation of zinc absorption, sources of zinc intake, dietary factors and mineral interactions affecting zinc bioavailability are reviewed in the present paper. This is followed by an overview of the biochemical functions of zinc in enzymes, gene expression, endocrinology, immunology and oxidative stress. General signs and metabolic consequences of zinc deficiency as well as excessive intake and toxicity of zinc are summarized. Furthermore, national and international dietary zinc recommendations and different methods to determine the zinc status are discussed.

Reconstitution of ceruloplasmin by the Cu(I)-glutathione complex. Evidence for a role of Mg2+ and ATP

The copper-glutathione complex (Cu(I)-GSH) efficiently acted in vitro as the source of Cu(I) in the reconstitution of apoceruloplasmin. Copper was found to reinstate in the various sites in a multistep process, with metal entry into the protein in a first phase, and a second step involving conformational changes of the protein leading to the recovery of the native structural and functional properties. This latter phase was found to be strongly facilitated by Mg2+ or Ca2+ and by ATP. Both Mg2+ and ATP had to be present for optimal reconstitution. These results may shed some light on the mechanisms governing the biosynthesis of ceruloplasmin in vivo. Cu(I)-GSH was the only complex able to reconstitute ceruloplasmin at neutral pH. Glutathione may thus function to shuttle the metal from the membrane copper pump, as the Wilson disease ATPase, and ceruloplasmin in the secretory compartments of the cell. The finding that ceruloplasmin acquires the native conformation after metal entry through a complex pathway triggered by Mg2+ and ATP suggests that they may act as physiological modulators of this process in vivo.

Relationship of lead-induced proteins to stress response proteins in astroglial cells

Astroglial cells are resistant to cell death and morphologic damage following lead (Pb) exposure at concentrations which elicit detrimental effects in neurons. A possible explanation may be that astroglial cells respond to Pb by increasing the expression of specific proteins, such as heat-shock proteins (HSPs), which confer resistance to low levels of Pb. However, there has been relatively limited information regarding the ability of Pb to evoke the synthesis of HSPs. In the current study, pulse-labeling of cultured astroglial proteins with [3H]-leucine was used to evaluate the nature of Pb-induced changes in protein expression. The effect of Pb on newly synthesized proteins was compared to the response elicited by heat-shock and oxidative injury. Immunoblot analysis was utilized to examine alterations in levels of various stress proteins including HSP27, HSP70, HSP90, and heme oxygenase-1 (HO-1). Even though Pb induced the synthesis of proteins with estimated molecular weights of 23 kDa, 32 kDa, 70 kDa, and 90 kDa, the accumulation of HSPs other than HO-1 was not observed. Hyperthermia and treatment with Na arsenite both resulted in enhanced expression of HSP70 and HO-1. In addition, exposure to hydrogen peroxide (H2O2), cadmium (Cd), and lipopolysaccharide (LPS) stimulated a rise in HO-1 levels. Although cellular insult failed to elicit an increase in either HSP27 or HSP90, cultured astroglia expressed readily detectable levels of both these proteins. Furthermore, Pb exposure resulted in the development of crosstolerance to subsequent injury by treatment with either Cd or H2O2. The results of this study indicate that Pb triggers a less conventional stress response in astroglial cells, which may provide enhanced resistance to the toxic effects of Pb.

Up-regulation of glutathione synthesis in rat kidney by methyl mercury. Relationship to mercury-induced oxidative stress

Prolonged exposure of rats to methyl mercury hydroxide (MMH) results, during the initial phase of exposure, in the rapid accumulation of mercury as Hg2+ by kidney cortex and in a significant increase in oxidative stress, as characterized by the rate of formation of thiobarbituric acid reactive substances (TBARS) by renal mitochondria. These events are accompanied by a progressive increase in steady-state levels of the mRNA encoding gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in glutathione (GSH) synthesis and a 2- to 3-fold elevation in renal cortical GSH levels. The present study showed that the increase in GSH content was accompanied by a concomitant decrease in the rate of TBARS formation. Subsequent to these initial phase events, continued MMH exposure was characterized by equilibration in the rate of renal Hg2+ accumulation, a sharp decrease in both the TBARS formation rate and GCS mRNA level, but sustained elevation of renal cortical GSH content. Depletion of GSH with buthionine sulfoximine subsequent to the decline in the rate of TBARS formation did not result in a rebound of the TBARS formation rate. These findings suggest that oxidative stress during the initial phase of MMH exposure is derived from the transformation of CH3Hg+ to Hg2+, which, in turn, induces the synthesis of Hg(2+)- and/or oxidant-scavenging GSH molecules via the up-regulation of renal GCS mRNA. The findings also suggest that resistance to Hg(2+)-mediated oxidative stress may be more closely associated with the capacity for up-regulation of GSH synthesis than with elevated GSH levels per se.

Heavy metal mediated inhibition of rBAT-induced amino acid transport

rBAT, a protein which is located in the brush border membranes of intestine and renal proximal tubule cells, was recently shown to induce electrogenic countertransport of neutral and dibasic amino acids after its expression in Xenopus oocytes. Here, we studied the effects of heavy metals on rBAT induced amino acid transport in Xenopus oocytes to clarify a possible involvement of rBAT in heavy metal-induced aminoaciduria. The heavy metals Hg2+ and Pb2+ inhibited rBAT-induced amino acid transport with a different profile of action. The Pb2+ mediated inhibition occurred rapidly upon superfusion and was readily reversible upon washout. The maximal inhibition caused by Pb2+ was about 50% of the amino acid-induced currents at an apparent affinity (Km) of about 10 microM. In contrast, the Hg(2+)-mediated inhibition occurred rather slowly, depending on its concentration, and was not reversible during washout with control solution. However, the Hg(2+)-mediated amino acid transport inhibition could be reversed with Hg2+ chelating agents and reducing compounds. Other oxidative agents, such as the membrane permeable 2,2'-Dithio-bis(5-Nitropyridine) (DTNP), but not the membrane impermeable 5,5'-Dithio-bis (2-Nitrobenzoic acid) (DTNB), mimicked the effect of Hg2+, and their effect could similarly be reversed with 2,3-Dihydroxybutane-1,4-dithiol (DTE). In conclusion, Pb2+ and Hg2+ inhibit rBAT-induced amino acid transport in a noncompetitive, allosteric fashion. Blockade of rBAT-induced amino acid transport may be involved in aminoaciduria following mercury or lead intoxication.

Metabolic coupling of glutathione between mouse and quail cardiac myocytes and its protective role against oxidative stress

Cultured quail myocytes were much more resistant to H2O2 toxicity than cultured mouse myocytes. The intracellular concentration of glutathione ([GSH]i) and the activity of gamma-glutamylcysteine synthetase (gamma-GCS) in quail heart cells were about five and three times higher, respectively, than in mouse heart cells, although catalase and glutathione peroxidase (GSHpx) activity was similar in both. Preloading of gamma-glutamylcysteine monoethyl ester (gamma-GCE), a membrane-permeating GSH precursor, increased the H2O2 resistance of cultured mouse myocytes. These observations suggest that the high [GSH]i and the high activity of gamma-GCS in quail myocytes are responsible for their high resistance to H2O2. Both H2O2 sensitivity and [GSH]i of mosaic sheets composed of equal amounts of mouse and quail myocytes approximated those of sheets composed entirely of quail myocytes. From these observations, it is hypothesized that GSH was transferred from quail myocytes to mouse myocytes, probably through gap junctions between them, and that quail myocytes resynthesized GSH by a feedback mechanism, thus maintaining their intracellular GSH levels. When the fluorescent dye lucifer yellow was injected into a beating quail myocyte in a mosaic sheet, it spread to neighboring mouse myocytes but not to neighboring L cells (a cell line derived from mouse connective tissue). These observations indicate that existence of gap junctions in the region of cell contact between mouse and quail myocytes but not between quail myocytes and L cells. When quail myocytes preloaded with [3H]gamma-GCE were cocultured with mouse myocytes and L cells, the radioactivity was transmitted to neighboring mouse myocytes but not L cells. These observations show that GSH and/or its precursors can be transmitted from quail myocytes to mouse myocytes through gap junctions and that this can protect mouse myocytes from H2O2 toxicity. Mouse myocyte sheets composed of 10(4) cells or more showed higher resistance to H2O2 toxicity than single isolated mouse myocytes. Metabolic coupling of GSH between myocytes may contribute at least in part to this high resistance of the cell sheets.

Glutathione mercaptides as transport forms of metals

Among the many cellular functions of GSH, the roles of this tripeptide in metal transport, storage, and metabolism have recently received considerable attention. Although these roles had often been overlooked, they are critical for normal cellular metabolism and for protection from xenobiotics. Indeed, a number of the protective and regulatory functions of GSH are related to its ability to chelate reactive metals. GSH functions in the mobilization and delivery of metals between ligands, in the transport of metals across cell membranes, as a source of cysteine for metal binding, and as a reductant or cofactor in redox reactions involving metals. However, the interaction between GSH and metals can also produce or exacerbate cell injury. For example, GSH appears to be involved in the renal accumulation and toxicity of a number of metals, and in the carcinogenicity of chromium. Additional work is clearly needed to identify the mechanisms involved, and to better define the roles of GSH in metal homeostasis.

Effect of zinc supplementation on resistance of cultured human skin fibroblasts toward oxidant stress

In purified system zinc has been shown to have an antioxidant role. Its effects on the resistance of cultured cells towards oxidative stress in vitro were examined. Diploid human skin fibroblasts were grown for 21 d in culture media (RPMI 1640 containing 15% fetal calf serum) added with different zinc (Zn) concentrations (100, 125, and 150 microM as Zinc chlorur ZnCl2). In comparison, cell controls were grown in standard culture media (6.5 microM Zn). The intracellular zinc levels of treated fibroblasts increased from 3- to 7-fold (2330 +/- 120 ng/mg protein in 150-microM Zn-treated cells versus 331 +/- 21 ng/mg protein in control cells). The intracellular copper increased 3- fold whereas the iron content slightly but not significantly decreased. The index of basal lipid peroxidation measured as thiobarbituric acid reactants (TBARs) of zinc-supplemented cells was lower than that of non zinc supplemented controls (0.89 mumol/g protein in 150 microM Zn-treated cells versus 1.59 mumol/g protein in controls). At these high doses of zinc, fibroblasts expressed lower antioxidant metalloenzymes activities. Diminished TBARs in Zn treated cells tends to support that Zn acts protectively against free radical mediated damage. However when the cells were challenged with extracellular oxidant stresses mediated by hypoxanthine/xanthine oxidase or hydrogen peroxide (H2O2), an increased toxicity in Zn-supplemented cells was observed. When we applied an intracellular oxidative stress as UV-B or UV-A radiation, Zn-treated fibroblasts were more resistant than cells grown in normal medium. If Zn has shown antioxidant effect in some in vitro or in vivo systems our observations clearly demonstrate that this role is not mediated by antioxidant metalloenzymes.

Combined effect of vanadium and zinc on certain selected haematological indices in rats

1. Two-month-old Wistar rats of both sexes received, as sole drinking liquid, an aqueous solution of ammonium metavanadate (AMV) and zinc chloride (ZC) at concentrations of 0.30 mg V/cm3 and 0.12 mg Zn/cm3 respectively, for a period of 4 weeks. 2. The reference groups received for drinking at this time: water, AMV or ZC solutions at the same concentration. 3. In all groups of animals there was a statistically significant decrease in the uptake of food, AMV or ZC, as well AMV-ZC solutions, as compared with the food and water taken up by the control group. 4. In the group of animals receiving AMV or AMV-ZC solution for drinking the body weight increment diminished significantly. 5. In the animals drinking the AMV-ZC solution a statistically significant decrease in the erythrocyte count and haemoglobin level in the peripheral blood were recorded, similar to the groups drinking AMV or ZC solution. 6. In rats drinking aqueous AMV or ZC solutions and in females receiving AMV-ZC solution the percentage of reticulocytes and polychromatophilic erythrocytes increased, moreover, in the peripheral blood. It was not, however, associated with marked percentage changes in the composition of the bone marrow cells.

Immunotoxic effects of mercuric compounds on human lymphocytes and monocytes. III. Alterations in B-cell function and viability

The major goal of the study was to determine the effects of high and low levels of mercury on human B-cells. Following treatment of B-cells with HgCl2 (0-1000 ng) and MeHgCl2 (0-100 ng), their activation by mitogens was evaluated. Both forms of mercury caused a dose dependent reduction in B-cell proliferation in the presence or absence of monocytes. MeHgCl was approximately 10 times more potent than HgCl2. Mercury also inhibited the ability of these cells to synthesize IgM and IgG. Analysis of the expression of activation markers indicated that CD69, an early marker of cell activation, was not effected by mercury. In comparison, B-cell expression of the low affinity IgE receptor and the transferrin receptor were significantly reduced. Of particular interest, cells activated by mitogen for 48 hr became refractory to the immunotoxic effects of mercury. When exposed to high levels of HgCl2 (0.5-10 micrograms/ml) and MeHgCl (0.05-1 micrograms/ml), there was minimal reduction in B-cell viability at 1-4 hr, however, after exposure to mercury for 24 hr, cell death was apparent. MeHgCl was approximately 5-10 times more potent than HgCl2. Electron microscopic analysis revealed early nuclear alterations characterized by hyperchromaticity, nuclear fragmentation and condensation of nucleoplasm. Both forms of mercury caused a rapid and sustained elevation in the intracellular levels of Ca++. The results of this investigation clearly show that mercury-containing compounds are immunomodulatory; moreover, the decrease in B-cell function indicates that this metal is immunotoxic at very low exposure levels. Furthermore, the cytotoxic events are consistent with the notion that mercury initiates changes associated with programmed cell death.

Enhancement of gamma-glutamylcysteine synthetase mRNA in rat kidney by methyl mercury

Glutathione (GSH), a major cellular antioxidant, is elevated 2- to 3-fold in kidneys of rats during prolonged treatment with mercury as methyl mercury hydroxide (MMH). Increased renal GSH is accompanied by a dose- and time-related elevation in the relative abundance of mRNA hybridizable to a cDNA probe which encodes renal gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis. Renal GCS mRNA is maximally elevated 4.4-fold at 3 weeks following initiation of MMH treatment. Enhancement of GSH and GCS mRNA content corresponds to a relative sparing of renal cells from oxidative tissue damage during MMH exposure. These observations suggest that increased synthesis of GSH at the genetic level occurs as an initial adaptive response to mercury-induced oxidative stress in kidney cells.

Primary cultures of rabbit renal proximal tubule cells. III. Comparative cytotoxicity of inorganic and organic mercury

The present study further developed primary cultures of rabbit renal proximal tubule cells (RPTC) as an in vitro model to study chemical-induced toxicity by investigating the comparative cytotoxicity of mercuric chloride (HgCl2) and methyl mercury chloride (CH3HgCl) to RPTC. Confluent monolayer cultures of RPTC exposed to HgCl2 and CH3HgCl for 24 hr exhibited a concentration-dependent loss in cell viability at culture medium concentrations greater than 25 and 2.5 microM, respectively. Vital dye exclusion was a more sensitive indicator of cytotoxicity than the amount of lactate dehydrogenase activity, alkaline phosphatase activity, N-acetylglucosaminidase activity, and protein content remaining on the culture dish. On the basis of vital dye exclusion, HgCl2 was less toxic to proximal tubule cells in culture than CH3HgCl after 24 hr of exposure, whether cytotoxicity was based on LC50 values (34.2 microM HgCl2 vs 6.1 microM CH3HgCl) or total cellular mercury uptake (4.6 nmol Hg2+/10(5) cells vs 1.25 nmol CH3Hg+/10(5) cells). Differences in the extent and rate of metal uptake were also evident. Maximum cellular uptake of Hg2+ occurred within 6-24 hr after exposure and was not concentration-dependent, whereas maximum uptake of CH3Hg+ occurred within 3 hr of exposure and was concentration-dependent. The intracellular distribution of both mercurials between acid-soluble and acid-insoluble binding sites also differed. At noncytotoxic concentrations of HgCl2 (0.04-5 microM), intracellular Hg2+ bound increasingly to acid-soluble binding sites as a function of time, from 15-30% after 6 hr of exposure to 40-60% after 72 hr of exposure. However, at subcytotoxic (25 microM) and cytotoxic (34.2 microM) concentrations, Hg2+ binding to acid-soluble binding sites remained constant at approximately 30-40% for 6, 12, 24, and 72 hr after exposure. In contrast, only 20% of total cellular CH3Hg+ was bound to acid-soluble binding sites after exposure to 0.039 to 6.1 microM CH3HgCl for 6, 12, and 24 hr. Total cellular glutathione content was unaffected after exposure to 0.04-5 microM HgCl2 and 0.039-6.1 microM CH3HgCl, but was depleted 6 hr after exposure to 25 and 34.2 microM HgCl2. These results indicate that CH3HgCl was a more potent cytotoxicant to RPTC in primary culture than HgCl2. Furthermore, compared to Hg2+, the low binding of CH3Hg+ to acid-soluble binding sites and the absence of a redistribution of CH3Hg+ from acid-insoluble to acid-soluble binding sites appeared to contribute to its more potent toxicity to cultured cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Protective effects of zinc on cultured rat primary hepatocytes to metals with low affinity for metallothionein

The purpose of this study was to determine if Zn pretreatment could protect rat primary hepatocyte cultures from the cytotoxicity of five metals that have little or no affinity for metallothionein (MT). Hepatocytes were grown in monolayer cultures for 22 h and subsequently treated with ZnCl2 (100 microM) for 24 h; which increased the MT concentration 15-fold. Following Zn pretreatment, hepatocytes were exposed to various concentrations of Mn, V, Cr, Se, or Fe for an additional 24 h. Cytotoxicity was assessed by enzyme leakage and loss of intracellular K+. The toxicity of all five metals was significantly reduced in the Zn-pretreated cells. Zn pretreatment had no appreciable effect on the hepatocellular uptake (1-24 h) of Mn or Se. Zn pretreatment also did not increase the distribution of Mn or Se to the cytosol and neither metal was bound to MT, suggesting the protection was not due to their binding to MT. However, Zn pretreatment significantly decreased Mn-, Cr-, and V-induced cellular glutathione depletion. In summary, Zn pretreatment of rat primary hepatocyte cultures protects against Cr-, Mn-, Fe-, Se-, or V-induced hepatotoxicity. This protection does not appear to be related to MT induction but may be due to Zn-induced thiol or membrane stabilization and/or other biological changes produced by Zn.

Immunotoxic effects of mercuric compounds on human lymphocytes and monocytes. II. Alterations in cell viability

The major goal of this investigation was to examine the cytotoxic properties of both HgCl2 and MeHgCl, in terms of their ability to alter human T-cell and monocyte viability. Following treatment with HgCl2 (0-20 micrograms/ml) or MeHgCl (0-2 micrograms/ml), there was minimal reduction in lymphocyte viability at 1-4 hr. However, after exposure to mercury for 24 hr, cell death was apparent. In comparison, monocytes exhibited significant loss of viability during the early exposure periods. MeHgCl was approximately 5-10 times more potent than HgCl2. Other indicators of cell death were also determined. Measurement of the energy charge ratio indicated profound changes in cellular energy conservation. Electron microscopic analysis of cells treated with mercury revealed early nuclear alterations characterized by hyperchromaticity, nuclear fragmentation and condensation of nucleoplasm. In concert with these nuclear changes, there was destruction of cytoplasmic organelles with loss of membrane integrity. Studies of phospholipid synthesis by mercury treated cells confirmed that there were alterations in membrane structure. Thus, there was a decrease in total phosphatide synthesis by treated cells. Moreover, monocyte phospholipid synthesis appeared to be more sensitive to the presence of mercury then lymphocytes. Finally, both forms of mercury caused a rapid and sustained elevation in the intracellular levels of Ca++. These morphological and biochemical changes are consistent with the notion that mercury initiates cytotoxic changes associated with programmed cell death.

Activation of cell membrane potassium conductance by mercury in cultured renal epithelioid (MDCK) cells

To elucidate mechanisms of mercury toxicity, the cell membrane potential has been determined continuously in cultured kidney (MDCK)-cells during reversible application of mercury ions to extracellular perfusate. Exposure of the cells to 1 microM mercury ions is followed by rapid, sustained, and slowly reversible hyperpolarization of the cell membrane, increase of cell membrane potassium selectivity, and decrease of cell membrane resistance. Thus, mercury ions enhance the potassium conductance of the cell membrane. Half maximal hyperpolarizing effect is elicited by approximately 0.2 microM. Higher concentrations of mercury ions (greater than 10 microM) eventually depolarize the cell membrane. At extracellular calcium activity reduced to less than 0.1 microM, 1 microM mercury ions still leads to a sustained hyperpolarization and increase of potassium selectivity of the cell membrane. As evident from fluorescence measurements, 10 microM, but not 1 microM mercury ions leads to a rapid increase of intracellular calcium activity. Pretreatment of the cells with either pertussis toxin or cholera toxin does not blunt the hyperpolarizing effect of mercury ions. In conclusion, mercury ions activate the potassium conductance by a mechanism independent of increase of intracellular calcium activity and of cholera toxin- or pertussis toxin-sensitive G-proteins. This activation of potassium conductance may account for early effects of mercury intoxication, such as kaliuresis.

The physiological role of zinc as an antioxidant

The purpose of this review is to consider whether an essential biochemical function of zinc (Zn) is to serve as an antioxidant. Zn has been shown to have an antioxidant role(s) in defined chemical systems. Two mechanisms have been elucidated; the protection of sulfhydryl groups against oxidation and the inhibition of the production of reactive oxygens by transition metals. Supraphysiological concentrations of Zn have antioxidant-like effects in organelle-based systems and isolated cell-based systems in vitro. Administration of pharmacological doses of Zn in vivo has a protective effect against general and liver-specific prooxidants. Dietary Zn deficiency causes increased susceptibility to oxidative damage in membrane fractions from some tissues suggesting that increased oxidative stress may be a small but significant component of the pathology observed in dietary Zn deficiency. However, the biochemical basis for Zn deficiency pathology remains unelucidated; critical antioxidant functions for Zn may still be uncovered.

The implication of renal glutathione levels in mercuric chloride nephrotoxicity

The effects of renal glutathione (GSH) depletion on renal injury following a single injection of mercuric chloride (HgCl2) were evaluated in the rat. Animals were injected with different doses of HgCl2 and the renal function were studied 1 h later. Diethylmaleate (DEM) (4 mmol/kg body wt, i.p.) induced a significant depletion of GSH by reducing renal GSH levels to 25% of control values. This effect lasted for 6 h. HgCl2-induced nephrotoxicity, as measured by fractional excretion of glucose, lithium, sodium, potassium and water was increased in rats treated with DEM. The time course of HgCl2 nephrotoxicity was also investigated by determining the renal function at different times after HgCl2 and HgCl2 plus DEM injection. Renal impairment was significantly more marked in rat depleted of GSH.